Optical surface figure refers to the deviation of an actual optic surface shape from the ideal surface shape. Traditionally, in optical systems that require optical elements with low deviation from the ideal, the optical elements are held in place using flexure mounts. For example, an optical element may be bonded using an adhesive to a flexure portion of the mount, where the flexure is formed from a metal alloy, such as invar. This arrangement is designed to reduce stress in the optical element that would otherwise occur if the optical element were fixed directly to the mount. Specifically, the flexure mount arrangement is used to reduce distortion of the element's optical surface due to thermal or mechanical deformation of the underlying mounting structure (e.g., when translating the mount).
However, the flexure mount/alloy interface arrangement can still lead to stress in the optical element, including stress from imperfections in mating of the flexure to the mount, stress from minor misalignments of the components in assembly, stress from deformations associated with cure-shrinkage/instability of the bonding material (e.g., epoxy) used to bond the optical element to the flexure mount, and stress arising from the different thermal expansion coefficients of the materials used. Moreover, when mounting optical elements (e.g., mirrors) having a tight figure tolerance and high aspect ratios, the mounting stresses can easily exceed tolerable levels, causing the optical element to bend out of the specifications required for an intended system.